The present invention relates to an air-fuel ratio control apparatus of an internal combustion engine. More particularly, the invention relates to an air-fuel ratio feedback control apparatus in which an air-fuel mixture having a base air-fuel ratio on the lean side of the desired air-fuel ratio is formed by a carburetor or the like disposed in an intake system of the engine and then the air-fuel ratio of the air-fuel mixture is controlled to the desired value by controlling the amount of fuel additionally supplied via at least one fuel injection valve disposed in the intake system, in accordance with an air-fuel ratio condition signal fed from an air-fuel ratio sensor, such as an oxygen concentration sensor, disposed in an exhaust system of the engine.
In conventional air-fuel ratio control apparatuses of this type, the air-fuel ratio control is performed by increasing or decreasing the amount of fuel additionally supplied from fuel injection valves by a certain quantity each time an injecting operation is carried out, based on a detection signal of an air-fuel ratio sensor. In this case, if the quantity of the fuel increased or decreased each time of the injecting operation is increased, the time required for obtaining the desired air-fuel ratio can be shortened, but after obtaining the desired air-fuel ratio, the amplitude variation of the air-fuel ratio is relatively large and it is very difficult to maintain the air-fuel ratio within a predetermined range approximating to and including the desired air-fuel ratio. Accordingly, the quantity of the fuel increased or decreased on each occurrence of the injecting operation is adjusted so that the amplitude variation of the air-fuel ratio is within the above-mentioned predetermined range. However, this method is defective in that a long time period from the start of the feedback control is required for obtaining the desired air-fuel ratio. Generally, in internal combustion engines of this type, a three-way catalytic converter capable of simultaneously purifying three pollutant components, HC, CO and NO.sub.x, is disposed in an exhaust system, and in order to maintain the purifying efficiency of this three-way catalytic converter at the highest level, the air-fuel ratio condition of an exhaust gas is controlled within a predetermined range. Accordingly, if the air-fuel ratio is on the lean side and such ratio is not controlled in a short time within the predetermined range even by starting the feedback control, pollutants of the exhaust gas, especially NO.sub. x, can not be purified; therefore, a problem concerning the legal regulation of emission is caused. Further, when an exhaust gas recirculation apparatus (EGR apparatus) is used in combination with the conventional air-fuel ratio control apparatus, if the air-fuel ratio does not reach the desired value but remains on the lean side, a problem is caused, in the operability of the engine, for example, the accelerability thereof, is extremely degraded.